Scientists struggle to explain how coronavirus moves through the air

A woman walks in Times Square wearing protective goggles, latex gloves and a face mask in New York City on Tuesday. Photo by John Angelillo/UPI | License Photo

April 9 (UPI) -- The novel coronavirus, COVID-19, can persist in the air for several hours and be carried 23 to 27 feet by gas clouds produced by coughs and sneezes, several studies suggest.

But the focus on whether a coronavirus transmission occurs primarily through large or small droplets is misguided, according to Lydia Bourouiba, who studies viral transmission at the Massachusetts Institute of Technology.

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Large droplets, expelled by sneezes and coughs, are thought to fall to the ground relatively quickly, which means they can contaminate or infect surfaces and people only within a range of a few feet. Smaller droplets, on the other hand, can become aerosolized and travel farther, or so the thinking goes.

But Bourouiba suggests such delineations aren't helpful. In her paper, published in the Journal of the American Medical Association, she points to research that shows swirling clouds of gas produced by coughs and sneezes can carry a wide range of droplet sizes some two dozen feet away.

"The rapid international spread of COVID-19 suggests that using arbitrary droplet size cutoffs may not accurately reflect what actually occurs with respiratory emissions, possibly contributing to the ineffectiveness of some procedures used to limit the spread of respiratory disease," Bourouiba wrote.

Some recent lab experiments -- detailed in the New England Journal of Medicine -- suggest that viral particles can remain in the air for several hours. But what relevance the experiments have to transmission investigations remains unclear.

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Studies of the real-world spread of the virus suggest COVID-19 can spread rapidly on cruise ships, even with passengers quarantined in their rooms. Researchers suggest lab experiments can help scientists better understand how the virus might behave in different environments.

"Does it explain it? I'd say no," Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh School of Medicine, told UPI in an email, referring to the implications of Bourouiba's research. "But does it help to understand? Yes. All well-controlled experiments help us understand transmission."

According to Qingyan "Yan" Chen, a professor of mechanical engineering at Purdue University and an expert in the spread of viruses through ventilation systems, studying the behavior of small virus-carrying particles, or aerosols, is essential to gauging risk and informing public health recommendations.

Chen's research suggests the potential for virus particles to travel through ventilation systems and circulate inside small, enclosed spaces makes some environs riskier than others.

"Since filters used in cruise ships are not highly efficient in filtering small droplets, coronavirus could be transmitted to other spaces in the ships," Chen told UPI. "However, airplanes use high efficiency particulate air, HEPA, filters. The small droplets containing coronavirus, if any, will be filtered out since HEPA filters can remove those droplets with 99 percent or higher efficiency."

Claus Wilke, a computational and evolutionary biologist at University of Texas at Austin, claims the usefulness of mechanistic coronavirus research -- studying virus particles in controlled lab settings -- has limited short-term use.

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When working to understand the transmission of a new virus, the advantage goes to the epidemiological approach -- who gets sick and under what circumstances, Wilke said.

"We know that 60 people can be in a room at choir practice, with no one displaying obvious symptoms, and 45 people can get sick," Wilke told UPI, referring to news out of Washington state.

"We don't have to understand how it happened, we know that happened. On the other hand, when people isolate in apartment buildings in Italy, they appear to be safe," he said.

Without lab tests, Wilke said, it's clear that being in close quarters with infected people, both asymptomatic and symptomatic, for extended periods of time is especially risky. Conversely, walking outside or living in apartments appears to be low-risk.

Wilke isn't opposed to mechanistic research. He and other scientists, including Paul Duprex, expect animal studies will offer scientists a better understanding of how close two people must be to one another to spread the virus.

"We are working on getting the optimal animal models of disease established so that shedding occurs and transmission can there be studied," Duprex told UPI.

"Once that's done it will be possible to study contact and aerosol transmission. We have done this for a very infectious virus called canine distemper virus, a cousin of measles virus," he said.

However, it's unlikely that mechanistic studies ever will pin down precisely how COVID-19 spreads. After decades of research, scientists still are arguing about how the flu is transmitted.

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Despite the uncertainty offered by early mechanistic studies of COVID-19, data analysis can offer us some sense of how easily the new coronavirus spreads.

"This virus is not actually as infectious as the measles, the classic airborne pathogen that spreads incredibly rapidly," Wilke said.

"On average, for each case that we have, we know how many new cases are caused by that. For the measles is 10. For the coronavirus, it's somewhere between two to three," he said.